Over the past several years, complex electronic devices such as computers and video cameras have become small enough to be easily portable. Portable, or "notebook", computers have become a popular alternative to traditional desktop computers, and small, hand held video cameras have become an ubiquitous household appliance. These portable devices are typically powered by rechargeable batteries or alternatively by a power adapter which can be connected to an AC outlet. Often it is desirable to operate the devices for lengthy periods of time in environments where AC power is unavailable, for example on airplanes or outdoors.
Though reduced in size, these complex electronic devices consume similar levels of power to their desktop predecessors. As a result, conventional rechargeable batteries, e.g., having a Nickel-Cadmium electrolyte, typically have a very short life--as little as one hour. Short battery life can be a severe constraint on the effective portable use of a device, for obvious reasons.
One known way to extend battery life is to carry extra charged batteries. The extra batteries are charged whenever AC power is available. Then, when AC power is not available, when a battery expires, it can be replaced with another freshly charged battery.
Most battery-powered devices include an internal charger for charging the battery; this charger will begin charging the battery whenever the device is powered by AC power. Thus, one method for charging extra batteries is to serially insert them into the device while the device is connected to AC power. However, this method can be inconvenient. First, the device may or may not indicate when the battery is charged; if there is no such indication the operator must leave each battery connected at least as long as the maximum possible charge time to ensure that the batteries are charged, even if a given battery requires substantially less time to achieve a full charge. Second, even if the device does indicate when the battery is charged, it is inconvenient to constantly monitor the device and replace batteries.
Due to these inconveniences, battery chargers have become popular. Battery chargers have an independent power supply and connectors for accepting one or more batteries to be charged. Thus, a charger can be an efficient way to charge extra batteries; the batteries are simply plugged into the charger, and some time later the batteries are charged.
Another known way to extend battery life is to use batteries having exotic electrolyte technologies which provide longer battery life. For example, longer-life batteries can be made using Nickel metal hydride and Lithium ion electrolytes. Batteries of this type have been found to provide substantially more battery life compared to Nickel-Cadmium electrolyte batteries.
However, these new battery technologies have the difficulty that the battery can be easily damaged by improper charging. Typically, these more exotic batteries must be charged in accordance with very specific voltage and current profiles; if these profiles are not followed, damage could result. Accordingly, these new electrolytes are typically only used with "smart battery" circuitry which includes, in essence, a computer circuit having the sole function of monitoring the performance of the battery and regulating both the charging and discharging of the battery to achieve optimum performance. This smart battery circuit continually communicates with the device or charger connected to the battery to achieve these goals.
While these new technologies have extended battery life, they have substantially increased the cost of battery chargers, because a charger for a smart battery must have sufficient circuitry to communicate with the smart battery and carefully control the charging and/or discharging of the battery to avoid damage. Furthermore, the manner of communication between the battery and charger is typically proprietary to the manufacturer of the battery, increasing the difficulty of making a compatible charger and often preventing third parties from manufacturing compatible chargers and thereby increasing costs to the consumer.
Although this expense can be avoided by simply using the charger built into the battery-powered device, this alternative involves the inconvenience of constantly monitoring the device and swapping the batteries, and, if the device does not indicate whether a battery is charged, may also require the operator to leave a battery connected to the device for a much longer period than is actually necessary to charge the battery.
Thus, particularly in the context of smart batteries, there is a need for a low-cost device for charging batteries which does not require constant monitoring or swapping of batteries, and which does not require the operator to leave a battery connected any longer than is necessary to charge the battery.